1. Field of the Invention
The present invention relates to a luminance and chrominance signal separating apparatus for separating a luminance signal and a chrominance signal from a composite color television signal.
2. Description of the Prior Art
According to recent digital signal processing techniques, an adaptive type luminance and chrominance signal separating apparatus selects a filter according to whether a correlation exists between a signal at a reference point and a signal in a horizontal or vertical vicinity of the reference point. This type of apparatus is used for separating a luminance signal and a chrominance signal from a composite color television signal.
A block diagram of a luminance and chrominance signal separating apparatus in accordance with the prior art is shown in FIG. 1.
An input composite color television signal is delayed at a first delay device (line memory) 401 and at a second delay device (line memory) 402 each by 1H period. Here, 1H means one horizontal period. Because each of a first subtracter 404 and a second subtracter 405 have two input signals having a 1H time difference, they perform as a comb filter. An output signal of the first 1H delay device 401 is regarded as a reference signal.
The output signal of the first delay device 401 is directly inputted to a first bandpass filter 403 and a chrominance signal is extracted there.
A correlation between the reference signal and the signals which are separated (ahead and behind) by each 1H period from the reference signal is detected at a first correlation detector 406 and a second correlation detector 407. The absolute value of the difference between two composite color signals (i.e., the reference signal supplied from the first delay device 401 and the signal which is the input composite color signal that is 1H period ahead of the reference signal) is determined. The absolute value is then filtered and the filtered value is compared to a specified value to detect whether a correlation exists or not. These three actions are performed separately at both the first correlation detector 406 and the second correlation detector 407.
The signal ahead of the reference signal by 1H period is located at the point on the upper scanning line adjacent to the point where the reference signal exists. The signal behind the reference signal by 1H period is located at the point on the lower scanning line adjacent to the point where the reference signal exists. The points on the upper and lower adjacent scanning lines can be called vertical vicinities (on the screen) of the point where the reference signal exists.
One of the three outputs (from the two subtracters 404 and 405 and the first bandpass filter 403) is selected at a selector 408 based on the output of the first correlation detector 406 and the second correlation detector 407. For example, when there is a correlation between the reference signal and the signal 1H period ahead of the reference signal, the output of first subtracter (comb filter) 404 is selected. When there is a correlation between the reference signal and the signal 1H period behind the reference signal, the output of the second subtracter (comb filter) 405 is selected. When there is no correlation both between the reference signal and the signal 1H period ahead of the reference signal and between the reference signal and the signal 1H period behind the reference signal, the output of the bandpass filter 403 is selected. An optimum filter can be selected by detecting a correlation. A chrominance signal is obtained at the output of a second bandpass filter 409 by limiting a color frequency bandwidth of the output of the selector 408. A luminance signal can be obtained by subtracting (using subtracter 410) the chrominance signal at the output of the bandpass filter 409 from the time compensated signal which is delayed by 1H period from the input signal.
In the above-mentioned configuration, however, when an incorrect filter is selected due to a misdetection at the correlation detectors, if the reference signal level is a black level, dot interference could be outstanding.
The present invention offers a luminance and chrominance separating apparatus in which dot interference cannot be perceived even if the correlation detectors misdetect a signal.